/Engine/Source/Editor/UnrealEd/Classes/ThumbnailRendering/ThumbnailManager.h
Writes out a png of what is currently in the specified viewport, scaled appropriately
static bool CaptureProjectThumbnail ( [FViewport](API\Runtime\Engine\FViewport) * Viewport, const [FString](API\Runtime\Core\Containers\FString) & OutputFilename, bool bUseSCCIfPossible )